Runtime register allocator

ABSTRACT

Methods and arrangements for accessing a storage structure. Included are an arrangement for providing a storage access instruction, an arrangement for inputting an address into a storage structure data cache responsive to a storage access instruction, an arrangement for extending a storage access instruction with a predicted register number field, the predicted register number field containing a predicted register number corresponding to a speculative location of a load/store operand associated with a storage access instruction, an arrangement for speculatively accessing a storage structure with a storage access instruction extended by the extending arrangement, and an arrangement for reverting to the arrangement for inputting an address if the load/store operand is not in the speculative location.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 10/732,656 filed on Dec. 10, 2003, now U.S Pat. No.7,290,092 the contents of which are hereby incorporated by reference inits entirety.

FIELD OF THE INVENTION

The present invention relates in general to memory access and increasingthe efficiency thereof.

BACKGROUND OF THE INVENTION

In general, the number of clock cycles needed for accessing memory hasbeen increasing. Solutions to this problem have indeed emerged over theyears, but have not yet proven to be adequate. For instance, differentcaching techniques have been contemplated that store a subset of memoryinto smaller structures and use the data address to decide where aspecific datum must be stored (see Smith, A. J., “Cache Memories”,Computing Surveys, September 1982). However, it has also been noted thatrecent technological trends of increasing wire delays will lead toeither increasing access times or decreasing capacities for caches (seeAgarwal et al., “Clock Rate Versus IPC: The End of the Road forConventional Microarchitectures”, Proceedings of the InternationalSymposium on Computer Architecture [ISCA], 2000). Generally, theconventional technique of looking up caches by address will either causeconflict misses or require an associative search that can increase cacheaccess time and consume excessive quantities of power. Register filestend not to present such problems.

Also, since the register number is a field which is part of theinstruction, the desired register can often be accessed as soon as theinstruction is fetched, leading to a short pipeline. This contrasts withcaches in the context of current instruction sets, which require that anaddress first be read from a base or index register, then possibly becomputed through an address addition, and then possibly be translated toa real address, before the cache access can start. Shorter pipelines canoffer well-known advantages, such as lower branch penalties.

However, register use is usually scheduled by compilers, which tend tomiss many opportunities for allocating memory locations into registers,because of the limitations of compile-time algorithms. For example,register allocation often requires that a compiler should prove that twopointers will never refer to the same memory location, which is hard todetermine precisely at compile time.

Generally, a register file represents the first line of defense inavoiding accesses to main memory. By filtering accesses to the level-1data cache (DL1), the register file reduces the number of accesses tothe memory hierarchy. It thereby allows better utilization of the cachesat all levels, thus conserving cache capacity and bandwidth for theinstructions with poor temporal locality that actually need it. Modernalgorithms for static register allocation succeed in servicing a largenumber of requests for data that would otherwise have to go to thememory hierarchy.

In spite of such sophisticated static algorithms, a large proportion ofaccesses to the DL1 exhibit temporal locality. While this is good forreducing the miss-rate of the DL1, it suggests that there is stilladditional room for filtering out accesses to the DL1.

In view of the foregoing, a need has been recognized in connection withimproving upon the shortcomings and disadvantages of prior efforts.

SUMMARY OF THE INVENTION

In accordance with at least one presently preferred embodiment of thepresent invention, there is broadly contemplated herein the enhancementof existing store-by-address memory hierarchies in systems, with dynamicstore-by-instruction techniques. In this latter approach, memoryoperands are accessed by short fields directly in the instruction,similar to register numbers. The proposed approach is dynamic, meaningthat throughout the lifetime of the program, these register-number-likefields in a given instruction may be changed dynamically. In its endresult, the proposed approach allows a higher level of associativity incaches, including fully-associative storage (i.e., whereby any datum canbe stored in any location) without requiring the frequent use ofexpensive associative searches. As a result, larger and more highlyassociative storage structures will become feasible at “aggressive”clock rates.

Generally, an aim adopted herein is to avoid associative search whileretaining the benefits of conventional cache line replacement policies.Towards this end, one may preferably perform location-predictionindependently for each load and store instruction and store thelocation-prediction bits along with the instruction in the I-cache andthe rest of the instruction memory hierarchy (In this context, thelocation of a load-store data operand in a set associative data cachemay be defined as a sequence of bits indicating <associative set number,way number within set, offset with line>; i.e. the location containssufficient information to access a load/store operand directly in acache array, without performing any address computation, addresstranslation or associative search).

Another aim adopted herein is to allow the data cache to be accessed asearly in the pipeline as the register file, by way of the aforementionedlocation-prediction. In the case where location-prediction fails, onemay preferably perform an associative search and access the cache in aconventional fashion. When location-prediction succeeds, however, thenthe process becomes quite similar to the way in which register files aretypically accessed. Particularly, the location to access is preferablyspecified by bits in the instruction, and a simple and direct lookup isperformed in the cache array, rather than associative search. Since theprediction is available immediately after instruction decode (and evenimmediately after instruction fetch, assuming the instruction encodingforces the critical location prediction bits to be always placed in thesame bit positions within an instruction), one can now access the loadand store operands earlier in the “pipeline”, thus mimickingregister-like access behavior.

In summary, one aspect of the present invention provides an apparatusfor accessing a storage structure, the apparatus comprising: anarrangement for providing a storage access instruction; an arrangementfor inputting an address into a storage structure data cache responsiveto a storage access instruction; an arrangement for extending a storageaccess instruction with a predicted register number field, the predictedregister number field containing a predicted register numbercorresponding to a speculative location of a load/store operandassociated with a storage access instruction; an arrangement forspeculatively accessing a storage structure with a storage accessinstruction extended by the extending arrangement; and an arrangementfor reverting to the arrangement for inputting an address if theload/store operand is not in the speculative location.

Another aspect of the present invention provides a method of accessing astorage structure, the method comprising the steps of: providing astorage access instruction; providing an arrangement for inputting anaddress into a storage structure responsive to a storage accessinstruction; extending the storage access instruction with a predictedregister number field, the predicted register number field containing apredicted register number corresponding to a speculative location of aload/store operand associated with a storage access instruction;speculatively accessing a storage structure with a storage accessinstruction extended via the extending step; and reverting to thearrangement for inputting an address if the load/store operand is not inthe speculative location.

Furthermore, an additional aspect of the present invention provides aprogram storage device readable by machine, tangibly embodying a programof instructions executable by the machine to perform method steps foraccessing a storage structure, the method comprising the steps of:providing a storage access instruction; providing an arrangement forinputting an address into a storage structure responsive to a storageaccess instruction; extending the storage access instruction with apredicted register number field, the predicted register number fieldcontaining a predicted register number corresponding to a speculativelocation of a load/store operand associated with a storage accessinstruction; speculatively accessing a storage structure with a storageaccess instruction extended via the extending step; and reverting to thearrangement for inputting an address if the load/store operand is not inthe speculative location.

For a better understanding of the present invention, together with otherand further features and advantages thereof, reference is made to thefollowing description, taken in conjunction with the accompanyingdrawings, and the scope of the invention will be pointed out in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates the execution of a load instruction inconnection with a conventional set-associative data L1.

FIG. 2 schematically illustrates an analogous process as in FIG. 1, butin accordance with an embodiment of the present invention.

FIG. 3 schematically illustrates the way how a special DL1 optimized foravoiding the alignment overhead for part-word operands can be accessed.

FIG. 4 schematically illustrates how the part-word operands are storedin aligned form inside the special DL1 optimized for avoiding alignmentoverhead for part-word operands.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

By way of general illustration, FIG. 1 shows a conventionalset-associative data L1 cache, and how a simple load instruction usesit. The load instruction L rt=(rb) (102) will access the register file(RF) 104 with index “rb” to get the virtual address of the load operand.The upper bits of the virtual address are then translated to thecorresponding real address 108 using a “translation lookaside buffer”106. The set associative L1 data cache array 110 is accessed as usual,with the real address. Within a cache set, an associative tag-matchcomparison will be made against all entries in that set. If the desiredcache line is not found in the L1 (or, as alternatively termed herein,DL1) cache, lower level caches will be searched, and the data will bebrought into the L1 cache, evicting and possibly casting out an existingline in the L1 cache, according to the replacement policy. Otherillustrated possible functions, as discussed heretofore, are: a read orwrite command (112), a reload/castout interface (114), a cache misssignal (116) and the importation/exportation of data (118).

In contrast, in accordance with at least one presently preferredembodiment of the present invention (as shown in FIG. 2), all L/STinstructions (202) may preferably be extended with a Predicted RegisterNumber (PRN) field 203. (Elements of FIG. 2 not mentioned here andotherwise analogous to those shown in FIG. 1 bear reference numeralsadvanced by 100.) The PRN comprises of the subfields <associative setindex, way number within set, and offset within line>, and can be usedto access the desired operand within the cache array directly, withoutperforming any associative search and without having to perform anyaddress computation or translation. First, the PRN is preferably sent tothe L1 cache 210. The access is speculatively completed quickly, as ifthe L1 were a register file, and subsequent operations that may dependon the load data are also started speculatively, as soon as possible. Ifthe L/ST operand is already in the cache array location denoted by PRN,then there is essentially nothing to be done; the access was correct.Otherwise, first, the speculatively started operations that may dependon the L/ST are “squashed” (canceled). The set-associative L1 cache 210is accessed as usual, using the load/store real address (e.g., using, asdiscussed similarly with respect to FIG. 1, a register file (RF) 204 and“translation lookaside buffer” 206). If there is a cache miss (216), thelower level cache(s) are preferably accessed as usual, and an existingline in L1 210 is evicted for replacement (e.g., casting it out to level2 [L2] if it was “dirty” or overwritten; the choice of line to beevicted can of course vary, according to the replacement policy ineffect). The load/store instruction is then preferably completed withthe correct operand in the L1 cache 210. Also, as indicated at 220, thecorrect current location of the load/store operand is preferably writteninto the PRN field 203 of load/store instruction 202 that caused thelocation misprediction.

By way of further elaboration, in accordance with an embodiment of thepresent invention, the size of load/store instructions is preferablyincreased via adding an extra field, “PRN” (predicted register number).This field denotes the predicted location of the memory load/storeoperand in the L1 data cache (DL1). The PRN field preferably includessubfields which indicate the precise location of a load/store operandwithin an n-way associative cache array directly, in the form:<associative set index, way number within set, offset within line>.

Preferably, when a load or store instruction executes, its PRN fieldwill be provided to the data L1 (DL1) cache as a prediction about wherethe operand is expected to be in the DL1 cache. The memory address ofthe load/store operand is also preferably sent to the DL1 cache alongwith the size of the operand. If this is a store request, then the datato be stored is also preferably sent to the DL1 cache.

If the operand is indeed where the PRN field predicts (i.e., (1) the tagof the DL1 cache line designated by the “associative set index, waynumber within set” fields of the PRN is valid and actually contains thetag subfield of the load/store operand address, and (2) the associativeset index subfield of the PRN matches the associative set index subfieldof the load/store operand address, and (3) the “offset within line”subfield of the PRN field matches the “offset within line” subfield ofthe load/store operand address), the data is simply returned on a loadrequest, or the operand in the DL1 cache is overwritten with the newlyprovided data on a store request.

If the operand is not where the PRN field predicts, the system willpreferably perform a look-up of the memory hierarchy starting from theDL1 cache in a normal manner, with misses in the DL1 causing accesses tolower levels in the hierarchy and resulting in cache line replacementsor cast outs according to the memory hierarchy policy in place. The newlocation of the operand (the <associative set index, way number, offsetwithin line inside the DL1) will preferably be returned by the memorysystem, and the instruction's PRN field will be overwritten with thisnew operand location within the DL1 cache. If this is a load request,the data is also preferably returned with the new PRN value.

Since instructions have fields that can be written at runtime, theinstruction L1 cache and instruction memory hierarchy will preferablypermit write-back.

Preferably, there will be two special values of the PRN field, namely,the non-sticky invalid value and the sticky invalid value. Thenon-sticky invalid value in the PRN field forces a misprediction (i.e.,does not match any cache line's tag) and thus results in a normal cachelookup, but the PRN field of a load/store instruction can be replaced bythe correct value after its first execution. This sticky invalid valuein the PRN field forces a misprediction and thus results in a normalcache access, but the PRN field cannot be replaced, so a load/store witha PRN field equal to the sticky invalid value always acts just like anormal load/store, without any operand location prediction.

When a program is first loaded in memory, the PRN fields of loads/storesare initialized to the non-sticky invalid value, so that during thefirst execution of a load/store a misprediction will occur and the PRNfield will then be set to the location of the load/store operand in theDL1 cache. Various static compile-time or online run-time algorithms canbe used to detect load/store instructions unsuitable for prediction (forexample, when an instruction continually loads or stores new addresses),so that its PRN field can be set to the sticky invalid value, forcingthe operation to behave like a normal load/store thenceforth,mispredicting on every execution. (See Calder et al., “A Comparativestudy of Load Speculation Architectures”, The Journal of InstructionLevel Parallelism 1, 2000, Section 3.2, for an online counter-basedalgorithm that can be used in this context. Similar compile-timetechniques may be designed in accordance with Wang et al., “GuidedRegion Prefetching: A Cooperative Hardware/Software Approach”,Proceedings of the International Symposium on Computer Architecture,June 2003, section 4.)

The PRN field is available as soon as the instruction is fetched, butthe actual load/store address may be take multiple cycles to compute ina typical processor, involving reading a base and/or index register,possibly performing an address addition, and possibly performing avirtual to real translation on the upper bits of the address. Inaccordance with at least one embodiment of the present invention, a loadinstruction can execute immediately after the instruction is fetched, byaccessing the location indicated in the PRN field, and then theoperations dependent on the load can also be started speculatively. If alocation misprediction is detected later, the operations dependent onthe speculatively executed load that were already executed can be“squashed” (canceled), and re-executed with the correct data.

For the case where it is known that location mispredictions will occurfrequently for a load (i.e., the PRN field is set to the sticky invalidvalue), loads can instead be optimized for the location mispredictioncase, by not performing the initial speculative access using the PRN,and scheduling independent operations while the longer latency load istaking place.

When predictions are correct, one can benefit from full associativitywith register file-like access costs. Assuming that predictions willoccur more frequently than mispredictions, a cache level whereprediction is being used can be made fully associative, with onlydirect-mapped access costs when the prediction is correct. When a cachelevel is fully associative, there is only one set, thus the “set index”subfield in the corresponding PRN field can be omitted, thus making thePRN include only <way number, offset within line>.

In implementing the write-back to PRN fields, the writable store forupdating the PRN field(s) of an instruction can be allocated inside theinstruction cache (I-L1 and possibly I-L2) as in the foregoingdescription, or it can be implemented as a separate read/write arrayaddressed by the program counter of a load/store instruction.

General registers and the data L1 cache can be unified. A PRN field hasproperties similar to a register index normally found in instructions;it allows register-sized fields in the DL1 cache to be read very earlyin a microprocessor pipeline, like general registers. The DL1 and thenormal register file can in fact be merged in the same hardware array byusing some special lines of the DL1 to store the ordinary generalregisters. This approach reduces load/store instructions where thepredictions are correct, into register-to-register copy operations inthe sense that the processing of these load/store instructions can beginas early in the “pipeline” as instructions involving only registers. Theprocessor's general register operations (such as r1=r2+r3) can also beextended to operate not only on general registers but also on PRNfields, which refer directly to memory operands in the DL1 cache, as ifthey were general registers.

The following actions can be performed separately from, and concurrentlywith, the general register operations that efficiently performarithmetic or logical operations on PRN fields:

-   -   (1) checking that the tag of the cache line given by a PRN        field, as well as the set number and offset subfields of the PRN        field, indeed match the load/store operand address;    -   (2) if there is a mismatch, looking up the operand at the given        address in the memory hierarchy;    -   (3) accessing lower level caches such as L2, L2's of other        processors, or the main memory and, if the operand is not found        in the L1, choosing a line to evict in the L1, and casting it        out if dirty (i.e., changed by a store);    -   (4) correcting the incorrect PRN fields in the instruction        stream; and    -   (5) re-executing the operations that used the incorrect PRN        fields and all of their already-executed dependent operations.

PRN field values can be reused to avoid rewriting instructions. Whenthere is an advantage to avoid rewriting the PRN field of a load/storeinstruction, the memory hierarchy cache line eviction policy can beoverridden to prefer using the same PRN field value for new addressesthat a load/store refers to, where this is possible. If the PRN fieldremains the same even if the address has changed, this would avoidrewriting of the instruction. Not rewriting instructions could havebenefits such as avoiding instruction cache port contention is someimplementations.

The approaches broadly contemplated herein can be implemented byextending the Instruction Set Architecture (ISA) visible to theprogrammer (as above), or in a manner completely transparent to theprogrammer, inside the microarchitecture, or as part of a binarytranslation system. When the approach is being implementedtransparently, the PRN fields of instructions will be created only inthe instruction cache hierarchy (and initialized to the non-stickyinvalid value) when the instructions are first read from memory, will beupdated while they exist in the instruction cache hierarchy, and will bediscarded when instruction cache lines are evicted from the instructioncache hierarchy.

An instruction may use more than one PRN field, and the datum can beaccessed in multiple predicted locations given by the PRNssimultaneously, using multi-ported cache data arrays. If a locationmisprediction is detected later on any of the PRN fields, all operationsthat depended on the incorrect PRN and that were already executed, canbe “squashed” (canceled). Execution can continue uninterrupted, if oneof the PRN fields provided the correct location prediction. For the casewhere all of the PRN fields caused a misprediction, it suffices to writethe correct location prediction in only one of the PRN fields of theinstruction, and to re-execute the load or store and all the operationsthat depended on the load or store, that were already executed.

It is to be understood that although the present discussion has focusedprimarily on data cache access, the embodiments of the present inventioncontemplated herein can also be applicable in a wide range of generalstorage structures, such as instruction caches, or unified caches, inaddition to data caches. For example, an indirect branch instructioncould include one or more PRN fields, for predicting the location of thebranch target instruction in an associative instruction cache.

The embodiments of the present invention contemplated herein can also beapplied to caches at lower levels, including L2 and L3 (here, L1 denotesthe highest level in the cache hierarchy). Load/store operands that arenot frequently accessed, can be kept at a lower level cache and neverbrought into the L1 or even L2, perhaps by specifying a “maximum cachelevel” as a new field in the load/store instruction, above which thedatum accessed by the load/store should not be brought. Such operandscan also be accessed faster by specifying a “Predicted Register Number”(PRN) field in the instruction, for accessing the speculative locationof the operand in the L2 or L3 cache directly and more rapidly. In amultiprocessor environment where each processor has its own separate(perhaps L2) cache, to prevent unnecessary data exchange between thecaches of different processors, the load/store operand can also be keptin another processor's cache, and the predicted location of a datum inthe multiprocessor system can be expressed as a “Predicted RegisterNumber” (PRN) field, comprising the <processor id, set number, waynumber within set, offset within line>. Using the predicted location ofa datum in a multi-processor system can speed up inter-processorcommunication.

The approaches contemplated herein can also be applied to a special datacache, optimized for fast access to part-word load/store operands.Particularly, ordinary general registers have the advantage of being offixed size (equal to the natural word size of the underlying machine,perhaps 4 or 8 bytes). The capability of using load/store operands thatare smaller than the natural word size (part-word operands) will requirean extra multiplexer delay (typically implemented as a byte rotation andmasking operation) while accessing the data array, as compared toregister file access. To circumvent this problem and to get closer tothe access speeds of register files, a special set associative data L1cache can be designed as follows:

The line size of the special DL1 will preferably be set to the naturalword size of the underlying machine (4 bytes in the example in FIGS. 3and 4).

FIG. 3 first illustrates how a load/store address is sent to the normalDL1 with 4 byte lines, and then to a special DL1 also with 4 byte lines,but optimized for accessing part-word operands efficiently. The exampleshere are for an 8K byte, 8 way associative data cache. High-endianaddressing is used throughout FIGS. 3 and 4.

Given the original load or store address, the low order (leastsignificant) 2 bits (304) will be removed from the end of the addressand appended to the beginning of the address before sending it to thespecial DL1 cache. Also, the operand size (e.g., 1, 2 or 4 bytes) (302)will be appended to the beginning of the address being sent to thespecial DL1 cache. Thus the tag portion of the address will not onlyinclude the upper bits (306) of the original address, but also theoperand size (302) and starting offset (304) within a word. The “setselect” bits (308) can be taken from their usual place, just to the leftof the low order 2 bits (304) of the original load/store address. Whenan operand of a given size is accessed from lower cache levels for thefirst time (using the normal load/store address) and brought into thespecial DL1 cache, it is right justified and zero-extended within theDL1 cache line (if smaller than 4 bytes). If there are other operandswithin the same set that overlap the present operand in any of theirbytes, they will be evicted from this set and written to the lower levelcache if they were “dirty” (overwritten). This ensures that only oneinstance of a given byte within a word will exist within the DL1, toprevent inconsistent states between multiple occurrences of the samebyte in the cache.

FIG. 4 illustrates how two 2-byte halfwords in location 0x200 (402) and0x202 (404) are brought into the special DL1, and right justified andzero extended (406 and 408). For example, once the halfword in location0x200 (402) is placed in the special DL1 cache (408), no furtheralignment overhead is needed while accessing this halfword in the cache.

When an operand is accessed and found in the DL1 cache, (i.e. theoperand size, and low order bits and the upper bits of the address arepresent within the tags of the set selected by the “set select” field ofthe address) the following happens: On a load hit, the operand is readfrom the cache without any further byte alignment. On store hit, therightmost “operand size” bytes of the selected DL1 cache line iscompletely replaced by the rightmost “operand size” bytes of the newstore datum.

When a line needs to be evicted and it was dirty (overwritten), it iswritten to the lower level cache using the original load/store operandaddress and size.

When the main idea of the present invention can be applied to such aspecial DL1 cache, then the PRN field in the Load/Store instructioncomprises only of <Set number, way number within set>. The offset withinline is always 0. Moreover, the speculative accesses become faster andmore register-like, since there will be no byte rotation and masking(byte alignment) overhead to accommodate operands less than the naturalword size, during the data access.

It is not difficult to optimize the same data cache to store signextended operands as well, to avoid performing sign extension duringloads that hit in the DL1 cache. For example, one could use a one-bit“sign extended” flag as an additional part of the cache tag.

It is to be understood that the present invention, in accordance with atleast one presently preferred embodiment, includes an arrangement forproviding a storage access instruction, an arrangement for inputting anaddress into a storage structure data cache responsive to a storageaccess instruction, an arrangement for extending a storage accessinstruction with a predicted register number field, an arrangement forspeculatively accessing a storage structure with a storage accessinstruction extended by the extending arrangement, and an arrangementfor reverting to the arrangement for inputting an address. Together,these may be implemented on at least one general-purpose computerrunning suitable software programs. These may also be implemented on atleast one Integrated Circuit or part of at least one Integrated Circuit.Thus, it is to be understood that the invention may be implemented inhardware, software, or a combination of both.

If not otherwise stated herein, it is to be assumed that all patents,patent applications, patent publications and other publications(including web-based publications) mentioned and cited herein are herebyfully incorporated by reference herein as if set forth in their entiretyherein.

Although illustrative embodiments of the present invention have beendescribed herein with reference to the accompanying drawings, it is tobe understood that the invention is not limited to those preciseembodiments, and that various other changes and modifications may beaffected therein by one skilled in the art without departing from thescope or spirit of the invention.

1. An apparatus for accessing a storage structure, implemented inhardware said apparatus comprising: an arrangement for providing astorage access instruction; an arrangement for inputting an address intoa storage structure data cache responsive to a storage accessinstruction; an arrangement for extending a storage access instructionwith a predicted register number field, the predicted register numberfield containing a predicted register number corresponding to aspeculative location of a load/store operand associated with a storageaccess instruction, wherein information contained in the predictedregister number field comprises: an associative set number; a way numberwithin set; and an offset within line; an arrangement for speculativelyaccessing a storage structure with a storage access instruction extendedby said extending arrangement; and an arrangement for reverting to saidarrangement for inputting an address if the load/store operand is not inthe speculative location; wherein the storage structure is a data cache,the speculative location is a speculative cache array location, and thepredicted register number field contains information to start thespeculative access as soon as the instruction is available, therebyprecluding a need to compute and translate a load/store address.
 2. Theapparatus according to claim 1, wherein the storage access instructionis a load/store instruction.
 3. The apparatus according to claim 1,whereby the storage access is complete if the load/store operand isfound in the speculative cache array location.
 4. The apparatusaccording to claim 1, further comprising an arrangement for writing thecorrect location of the load/store operand into the predicted registernumber field of the storage access instruction.
 5. The apparatusaccording to claim 1, wherein the storage access instruction is aload/store instruction.
 6. The apparatus according to claim 1, wherebythe storage access is complete if the load/store operand is found in thespeculative location.
 7. The apparatus according to claim 1, furthercomprising an arrangement for writing the correct location of theload/store operand into the predicted register number field of thestorage access instruction.
 8. The apparatus according to claim 1,wherein said arrangement for inputting an address comprises of at leastone of: a register file and a translation lookaside buffer.
 9. A methodof accessing a storage structure, implemented in hardware said methodcomprising the steps of: providing a storage access instruction;providing an arrangement for inputting an address into a storagestructure responsive to a storage access instruction; extending thestorage access instruction with a predicted register number field, thepredicted register number field containing a predicted register numbercorresponding to a speculative location of a load/store operandassociated with a storage access instruction, wherein informationcontained in the predicted register number field comprises: anassociative set number; a way number within set; and an offset withinline; speculatively accessing a storage structure with a storage accessinstruction extending via said extending step; reverting to saidarrangement for inputting an address if the load/store operand is not inthe speculative location; wherein the storage structure is a data cache,the speculative location is a speculative cache array location, and thepredicted register number field contains information to start thespeculative access as soon as the instruction is available, therebyprecluding a need to compute and translate a load/store address.
 10. Themethod according to claim 9, wherein the storage access instruction is aload/store instruction.
 11. The method according to claim 9, whereby thestorage access is complete if the load/store operand is found in thespeculative cache array location.
 12. The method according to claim 9,further comprising an arrangement for writing the correct location ofthe load/store operand into the predicted register number field of thestorage access instruction.
 13. The method according to claim 9, whereinthe storage access instruction is a load/store instruction.
 14. Themethod according to claim 9, whereby the storage access is complete ifthe load/store operand is found in the speculative location.
 15. Themethod according to claim 9, further comprising an arrangement forwriting the correct location of the load/store operand into thepredicted register number field of the storage access instruction. 16.The method according to claim 9, wherein said arrangement for inputtingan address comprises of at least one of: a register file and atranslation lookaside buffer.
 17. A program storage device readable bymachine and implemented in hardware, tangibly embodying a program ofinstructions executable by the machine to perform method steps foraccessing a storage structure, said method comprising the steps of:providing a storage access instruction; providing an arrangement forinputting an address into a storage structure responsive to a storageaccess instruction; extending the storage access instruction with apredicted register number field, the predicted register number fieldcontaining a predicted register number corresponding to a speculativelocation of a load/store operand associated with a storage accessinstruction, wherein information contained in the predicted registernumber field comprises: an associative set number; a way number withinset; and an offset within a line; speculatively accessing a storagestructure with a storage access instruction extending via said extendingstep; reverting to said arrangement for inputting an address if theload/store operand is not in the speculative location; wherein thestorage structure is a data cache, the speculative location is aspeculative cache array location, and the predicted register numberfield contains information to start the speculative access as soon asthe instruction is available, thereby precluding a need to compute andtranslate a load/store address.